Restore the common versioned envelope for validate count JSON

[Widthdom]

Summary

Validate count JSON is only {"count":6,"total_estimated":6} and omits api_version plus common scope/freshness/completion metadata.

Audit IDs: R-11. Reproduced with locally built cdidx v1.40.3 at 404db09489c305f17d94f37026e00564d3ac527a.

Existing issue relationship

Regression/follow-up to closed #4579's validate output contract.

Expected behavior and PR scope

Count-only validate output should use the shared versioned response envelope without changing count semantics. Keep this to validate count serialization.

Implementation guide for Codex 5.6 sol xhigh

• Route count mode through the canonical validate response writer/DTO.
• Define exact versus estimated count, scope/filters, readiness/freshness, and completion fields.
• Cover empty/non-empty, limits, actionable filters, JSON modes, old DB readiness, and human count.
• Preserve scripts through documented compatibility or additive fields.
• Add focused positive/negative regression tests, run the full net8.0 and net9.0 suite, and add the required bilingual changelog.d/unreleased/ fragment.

Acceptance criteria

• Count JSON includes api_version and common metadata.
• The numeric count agrees with full validate under the same filters.
• Estimated/incomplete states are explicit.